1. Field of the Invention
The present invention relates to an electronic scanning radar apparatus and a received wave direction estimating method, which can detect a target using a reflected wave from the target in response to a transmitted wave and which can be suitably used for a vehicle, and a received wave direction estimating program used therein.
2. Description of Related Art
Electronic scanning type radars such as an FMCW (Frequency Modulated Continuous Wave) radar, a multi-frequency CW (Continuous Wave) radar, and a pulse radar have been known for some time.
In the radars, an arrival wave direction estimating method of an array antenna is used as a technique of detecting an arrival wave (or a received wave) from a target (a reflecting object).
Japanese Patent Application Nos. JP-A-2006-275840 (patent document 1), JP-A-2009-156582 (patent document 2), and JP-A-2009-162688 (patent document 3) describe that as an arrival wave direction estimating method using array antennas, high-resolution (high-accuracy) algorithms such as an AR spectrum estimating method (FIG. 47) which can obtain high resolution without increasing the number of channels of a receiving antenna and a MUSIC (Multiple Signal Classification) method have been used. The AR spectrum estimating method may be called a maximum entropy method (MEM) or a linear prediction method.
When an arrival direction (direction of Arrival: DOA) of a received wave from a target (a reflecting object) is estimated using such algorithms, the estimation process is performed after input data (data in which a noise component is mixed into a complex sinusoidal wave) expressed by a complex number is transformed into a matrix format of a correlation matrix. An arrival direction may be referred to as a DOA (Direction of Arrival).
For an arrival wave direction estimating method used in an in-vehicle radar, a correlation matrix in a past control cycle is stored and is subjected to an averaging process (or an adding process) with a correlation matrix in the present control cycle and then a direction estimating process is performed, to suppress a noise component and to improve estimation accuracy. Japanese Patent Application No. JP-A-2009-156582 describes that an averaging process of the correlation matrix provides a large improvement in a direction detection accuracy (azimuth accuracy or separation capability).
In addition, Japanese Patent Application JP-A-2009-162688 describes that such algorithms have a drawback in which the estimation should be performed after an appropriate number of arrival waves (received waves) (model order in the case of the AR spectrum estimating method) is set. It is necessary to set an appropriate value when the algorithm is applied to an in-vehicle radar.
With respect to the AR spectrum estimation method, the algorithm is performed by relatively light computational load, and which can perform an azimuth estimation process for a number of targets existing in a distance direction. Thus, the AR spectrum estimation method is suitable for in-vehicle radars. Furthermore, in comparison with the MUSIC method, the AR spectrum estimation method has an advantage, which can perform estimation without sensitively setting of the received wave number (a number of model orders), and the estimation accuracy improves as the number of model orders increases.
On the other hand, in the AR spectrum estimation method, when the number of received waves is much smaller than the number of settled orders and the noise component is a large number, there is an issue which a fail peak may be detected even if the averaging process is performed for the correlation matrix (normal equation) to improve accuracy.
Furthermore, for use as in-vehicle radar, which needs small size and low cost, the configuration of receiving system is required to design for less channel (CH) number, and for a case where the configuration is designed suitable for less channel (CH) number, the number of orders to be set becomes limited. In addition, even if the CH number of receiving system can be reduced, both setting a great number of orders and establishing a suitable setting in response to the number of signal receiving waves can be insufficient at the same time, because only setting a number of orders is possible for the AR spectrum estimation method (defined as a standard AR spectrum estimation method).
As a general estimation method of receiving wave number, an FPE (Final Prediction Error) method, an AIC (Akaike Information Criterion) method, an MDL (Minimum Description Length) method and the like are described, which determine a number of receiving waves. However, these methods require heavy computational load and any of these methods do not assure to provide high accuracy estimation result. There are few reports on research examples with respect to a small number of channels and a range of low orders for use of in-vehicle radar.